This invention relates generally to color picture tubes having three electron guns placed in line, and more particularly, the present invention relates to an improvement on a deflecting yoke of such a picture tube.
As is well known, three electron beams emitted from red, green and blue electron guns of a picture tube used in a color TV set or a color display are required not only that each of these beams is focussed but also converged at the phosphor screen. In a conventional color picture tube having three electron guns, which are arranged in a regular triangle or delta form, vertical and horizontal deflection magnetic fields are uniformly arranged for the three electron beams, and a convergence adjusting device for controlling the convergence of the three electron beams on the phosphor screen is employed so that the three electron beams are satisfactorily converged at any points on the phosphor screen. However, as the tendency of increasing the deflecting angle becomes more pronounced, it has been found that the conventional dynamic convergence assembly cannot achieve satisfactory dynamic convergence for corner portions of the screen. In order to solve this problem, many techniques and inventions have been developed hitherto as described in Japanese Patent Publication No. 52-33449 and others.
In the conventional color picture tubes having three electron guns which are arranged in a regular triangle form, utilization of a convergence adjustment device is essential for effecting dynamic convergence, and therefore it has been difficult to reduce the manufacturing cost.
Recently, a picture tube having three electron guns arranged in line in which self-convergence is effected has been provided, where the dynamic convergence of the three electron beams from the in-line electron guns is automatically performed by a pincushion horizontal deflecting magnetic field made by a pair of horizontal deflecting coils of a deflecting yoke, and by a barrel the vertical deflecting magnetic field made by a pair of vertical deflecting coils of the deflecting yoke. According to this technique, since no convergence adjusting device is needed, circuit arrangement can be simplified while cost reduction can be readily achieved, and thus this technique has been widely adopted to various devices using a color picture tube.
In the above-mentioned in-line type picture tube using the self-convergence system, the positional relationship between the magnetic field and the electron beams is changed by the horizontal and vertical deflection magnetic field made by the deflecting yoke attached to the picture tube so as to obtain a satisfactory state of convergence with the axes of the deflection magnetic field and the electron beams being aligned. However, when the deflection angle is as large as 90 degrees, there arises a problem that satisfactory state of convergence cannot be obtained. Namely, when it is intended to obtain a magnetic field distribution of the deflecting field so that pincushion distortion and barrel distorion are minimized, a conventional way of adjustment called neck-swinging adjustment, in which the open portion at the front of the deflecting yoke is moved up and down and left and right with the neck thereof fixed, cannot provide sufficient convergence.
When it is tried to improve misconvergence of positive crossing at the top and bottom of raster in a 90-degree deflection tube of relatively small size, such as 12 or 14-in, by changing the magnetic field distribution of the deflecting yoke, the reproduced image will deteriorate due to pincushion distortion at the top and bottom of the raster.
Since it is difficult to form a deflection magnetic field having a magnetic field distribution, with which both the form of raster and the state of dynamic convergence are brought into satisfactory condition as the fact that distortion occurs in raster when the magnetic field distribution of the deflecting yoke is changed to obtain satisfactory convergence, in conventional in-line type color picture tubes of small size, such as 12 or 14-in, a pincushion distortion compensating circuit has been employed for compensating for the pincushion distortion which occurs at the top and bottom of raster, although it resulted in increase in cost.
However, in an in-line type color picture tube used for graphic display, character display or the like in which it is required to change the scanning frequency, the pincushion distortion compensating circuit has to be adjusted in accordance with the change of the scanning frequency. Although such adjustment may be manually performed, it is very troublesome to do so, while it is also inconvenient for the user. When a circuit for automatically performing such adjustment is added to the pincushion distortion compensating circuit, it results in a high manufacturing cost.
Although a technique of attaching permanent magnets to the top and bottom of the deflecting yoke has been proposed for the improvement of the pincushion distortion and convergence, this technique cannot be applied to an in-line type color picture tube having dot type or perforated shadow-mask, which are used for providing images of high precision, because satisfactory purity cannot be obtained due to the use of the above-mentioned magnets.
On the other hand, in an in-line type picture tube of large size, such as 22 to 26-in, misconvergence of so called negative-crossing occurs on the convergence of electron beams at the top and bottom of the raster, and this raster distortion and convergence cannot be satisfactorily improved, lowering the quality of the reproduced images.
Furthermore, depending on the combination of a picture tube and a deflecting yoke, a large deviation or misconvergence of positive crossing occurs at a middle portion on the reproduced image, where a portion between the top and horizontal center line or between the bottom and the horizontal center line is meant by "middle portion". When the amount of deviation in convergence is greater at the middle portion between the top and the center or between the bottom and the center than that at the top or bottom, satisfactory convergence cannot be expected when conventional countermeasure has been applied.